In particular, the invention relates to a motor vehicle combustion engine.
The invention advantageously applies to internal combustion engines running either on gasoline or on alcohol, for example ethanol, or on a variable proportion of these two fuels.
As pollution-reducing standards become increasingly strict, it is necessary to check the combustion of the fuel in motor vehicle engines in order to have control over the pollution emitted by these engines. It is therefore necessary to monitor the fuel supply system of these engines in order to alert the driver, using a warning lamp, to any failure of the system which will cause the pollution thresholds to be crossed.
Such monitoring may be disrupted when the fuel used contains a variable alcohol content. Further, the phenomenon known as “blow-by”, whereby fuel vapors mix with the oil vapors from the engine oil sump, has a not-insignificant effect on the monitoring of the operational state of the fuel supply system. Further, the higher the alcohol content of the fuel, the more significant this blow-by phenomenon becomes.
At the present time, fuel supply circuit diagnostics are based on the monitoring of various parameters used to regulate the richness of the exhaust gases entering a catalytic converter on the basis of information delivered by a richness probe, also known as a lambda probe. What is meant by the “richness of the exhaust gases” is the quantity of oxygen present in the exhaust gases. In the case of an engine running on a single type of fuel, monitoring the variation in these parameters gives an idea of the level of failure of the fuel supply circuit. In the case of an engine which runs on a fuel containing a variable alcohol content, the parameters used to regulate the richness of the exhaust gases vary, not because of system failure, but because of the variation in the alcohol content. Specifically, the variation in the alcohol content leads to a variation in the amount of fuel that has to be injected in order to keep the richness on the outlet side of the engine constant for a given engine operating point. That being so, the monitored parameters vary without there being in any way any degradation in the fuel supply circuit, so the risk of detecting a failure state as a result of a change of fuel is therefore very high.
Further, the regulation used to set the richness of the exhaust gases can differ widely according to the type of fuel, because the corrector that regulates the richness is set according to the alcohol content of the fuel.
However, at the present time, diagnostics do not take the alcohol content of the fuel into consideration and so whatever the alcohol content used, their reliability is therefore not guaranteed.
Mention may be made of French patent application FR2892769, filed in the name of the Applicant Company, which describes a method for recognizing an alcohol content of the fuel of a motor vehicle on the basis of the measurement of the richness of the exhaust gases, which measurement is made by a richness probe. However, that document does not describe a means for diagnosing the operational state of the fuel supply system of the vehicle.
There is another problem that occurs when using alcohol-containing fuels. When the alcohol content of the tank is high enough, some of the alcohol injected enters the oil of the engine oil sump, and then under certain conditions, the fuel evaporates and enters the intake manifold. This phenomenon has the effect of disrupting the exhaust gas richness regulation, the parameters used for monitoring the fuel supply circuit, and therefore the diagnostics of the operational state of the engine fuel supply system.